1. Field of the Invention
The present invention is generally concerned with ophthalmic instruments and surgery, and more particularly relates to systems, methods, and apparatus for sensing and/or tracking the position of a human eye. The present invention is particularly useful for tracking the position of the eye during laser eye surgery, such as photorefractive keratectomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), or the like. In an exemplary embodiment, the present invention is incorporated into a laser ablation system to modify the distribution of laser energy directed at the cornea based on the sensed position of the eye during the laser ablation procedure.
The ability to track or follow the movement of a patient's tissue is recognized as a highly desirable feature, particularly for use in laser delivery systems designed to effect precision surgery in delicate ocular tissue. The eye movements to be tracked include not only the voluntary movements (which can be damped with specialized treatment), but also the involuntary movements which are more difficult to control in a living patient. In other words, even when the patient is holding "steady" fixation on a visual target, eye movement still occurs. This involuntary motion may compromise the efficacy of some ocular surgical procedures, which generally require a rate of precision. In fact, such involuntary movements may occur despite the "total immobilization" of the eye, as such techniques are not fully effective in suppressing involuntary eye motion, and are also rather uncomfortable for the patient. Automatic tracking of the eye may alleviate any need for this uncomfortable immobilization, and may offer a method for more effectively accommodating differing types of eye motion. In other words, augmenting surgery with real time eye tracking may improve the accuracy and speed with which known laser eye surgery can be performed, and may also enable new procedures to be carried out for the first time.
A variety of techniques have been described for tracking eye movements. One general type of eye tracking technique has been called "optical point tracking." Optical point trackers utilize various lens-like properties of the eye to locate optically distinguishable locations (for example, the first, second, third, and fourth Purkinje points). Unfortunately, such optical point trackers implicitly assume that the eye moves as a rigid body. As the eye actually flexes during movement, transient relative motions of lens structure can lead to fictitious optical point position information. In addition, optical point tracking systems are rather complex, and may exhibit large variability between individuals.
Another class of eye tracking techniques generally involve digital correlations and/or pattern recognition. These digital techniques generally require very fast frame-rate CCD cameras and sophisticated processing algorithms. These methods are fundamentally digital, and they generally involve very high frequency update rates. As tracking frequency response is considerably slower than update frequency in digital systems, they tend to be relatively slow. Regardless, digital methods generally do not provide continuous resolution, and often require extremely fast repositioning mechanisms to leave time for complex electronic processing within an acceptable total response time.
A recent promising technique for tracking eye movements takes advantage of the difference in the light scattering properties of the iris and sclera. In this technique, light is projected on to the iris/sclera interface or limbus, and the scattered light is detected by photodetectors to determine the boundary location. The relative position of this boundary can then be monitored to track the position of the eye.
Unfortunately, the limbus is more a transition zone between the cornea and the sclera, rather than a sharp boundary. As a result, techniques which rely on edge detection may lack the desired accuracy, and may not be capable of tracking large amplitude movements of the eye. Another disadvantage of known limbus tracking techniques is the relative complexity of signal processing required to effect tracking. In other words, when the eye moves so that the limbus is no longer in the nominal position, effecting realignment using known tracking systems requires fairly complex manipulations of the photodetector signal to properly instruct the repositioning system. These complex signal manipulations increase overall system complexity, and also slow the system down. Work in connection with the present invention indicates that slow tracking system response and less than desirable accuracies may in-part be the result of tracking system non-linearities. While adequate tracking response may be possible using known "pin-point" limbus trackers with accurately aligned photodetectors disposed precisely along the edge of the iris/sclera interface, providing and/or maintaining such alignment adds additional system components and complexity, particularly in light of the variability of eye geometry between differing patients.
In light of the above, it would be desirable to provide improved eye sensing and tracking devices, systems, and methods. It would be particularly desirable if these enhanced techniques improved tracking response times and sensitivity, but without significant increases in cost or complexity of the tracking mechanism. It would be particularly desirable to provide these enhanced capabilities in a system which was adaptable for use in laser eye surgery for accurately sensing and/or tracking a variety of patient eye movements.